Hi All
I hit the range yesterday for the first time in a while. One focus for the day was to sight in a new to me 26-06 encore barrel. I traded for it last spring and recall that the previous owner had purchased it, but then pivoted to a 243 without even firing it. The condition of the barrel supported that story.
On paper at 50 yards it was grouping 75gn vmax bullets at ~ 1 inch after a few sighters. When I moved to 100 yards, it grouped ~8 inches and wouldn't put 100gn pills on the paper. Back to 50 yards to get it on paper with 100gn TSX, then back to 100 and finding ~10-12" groups. In the end, 100gn TTSX, TSX, Nosler BT and 75gn vmax all patterned like a turkey choke at 100 yards. The last 3 shots of the day were some Hornady lead tipped spitzer bullets which actually grouped ~1.5 inches.

I fiddled with the scope throughout the day (Burris 3-12) and it didn't seem like the scope was loose. The last 3 shots indicate that it likely isn't the scope moving around, but I think i will pull the mount and check for burrs before swapping to a leupold off another barrel. I was also going to clean the barrel to see if there was anything strange there and then take it back to the range. The muzzle didn't have any noticeable burrs or dings. Maybe a burr inside some where? Any one have any other suggestions or things I'm missing (other then the target)?

I don't see it in your post but were any of the bullets keyholing at all? I had a .223 Super 14 do the same group wise but I noticed a few tumblers. A good cleaning fixed it up.

Some of the holes in the paper were oblong (75g Vmax only), but when I pushed the paper back into place, the darkened paper that made contact with the bullet was a circular ring. It appeared that it had just ripped out more on one side. I've tumbled bullets before and this did not match that observation.

What is the twist rate?

The internet says the factory barrel is a 1:10 twist (this is factory). I haven't looked to confirm.
The 75gn Vmax bullets were just as dispersed as the 100gn, but i was considering some 90gn loads if a good cleaning and scope reinstall didn't fix it.

Karl, oddly, I've had similar problems with my factory .25-06 Encore barrel. I've tried many powders and bullets. Every time I think I have it figured out, I take it back to the range a few days later and the shotgun pattern emerges. I've only used bullets from 100-120 grains from Horn., Sierra, Nosler, Speer, Barnes. All get weird.
The only powder that does half way decent is IMR 4831.
I think the gun has a jinx on it.

I have a factory Pro-Hunter 25/06 Pistol barrel that I bought second hand from an Idaho Pawn Shop. My experience has been quite the opposite. The barrel appeared have been shot very little by its previous owner, if at all. I shoot primarily 115-120 gr Nosler bullets and Federal's 115 gr. blue box factory load in which I believe the bullet is made by Speer. My primary powders are Accurate 4350, RL-25, RL-19, and H-4831SC. All of these powders are excellent shooters, and extremely accurate in my pistol. The only problem that I had initially was high pressures with certain loads due to the extremely short chamber leade. SAAMI MAX OAL is-3.250. Max length that I can load is 3.238 with 115 gr. BT, which has quite a bit of taper. Once I shortened up the loads a bit accuracy improved. Another thing that keeps accuracy consistent is the Burris 3 x 12 Ballistic Plex scope clamped to a Warne 6 screw base by four Warne Maxima rings. Consistently solid from year to year.

The only other thing I could suggest is your scope mount system.

Agreed. In addition, I have measured the breech gap on on mine, glass bedded the forend, and installed HD Locking bolts. I do not switch barrels between frames on my Encores, as I personally feel that practice can lead to inaccuracy. My opinion, but it works for me.

Thanks All! I'll do some investigating and see what I find.

Sooooo....
It appears that the cause of the problem was stupidity on my part.
I used an integrated TC scope base/rings on a Burris 3-12 scope. After closer examination it appears that the scope shifted ~8mm until the front bell contacted the front ring Most of my other guns have 3 rings on a steel base, but I brain farted when I ordered the integrated system. Now that the scope has stopped moving the barrel is grouping well enough to use this fall.

That's why I go with Warne on just about everything with noticeable recoil.

Glad you figured it out and thanks for letting us know.

Good luck this fall!

John